Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
A lithographic process, as described above, is performed to selectively remove portions of a resist material overlaying the surface of a wafer, thereby exposing underlying areas of the specimen on which the resist is formed for selective processing such as etching, material deposition, implantation, and the like. Therefore, in many instances, the performance of the lithography process largely determines the characteristics (e.g., dimensions) of the structures formed on the specimen. Consequently, the trend in lithography is to design systems and components (e.g., resist materials) that are capable of forming patterns having ever smaller dimensions.
Inspection processes based on optical metrology are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry implementations and associated analysis algorithms to characterize device geometry have been described. However, it remains a challenge to preserve a small measurement box size. A small measurement box size is especially important in semiconductor inline product metrology where the area available for metrology targets is minimal. The measurement box size refers to the minimum area on the specimen where measurement results are stable and not affected by edge effects (e.g., due to optical diffraction wings) in optical metrology. Hence, the smaller the measurement box size, the smaller the area required for metrology targets. In the semiconductor industry, where wafer space allocated to metrology targets is limited (often, within the scribe line or even within die), the desired box size specification can be often very challenging, such as 30 μm×30 μm, 10 μm×10 μm, or even smaller.
Diffraction, aberration, image quality, and other limiting effects must be controlled to achieve a smaller illumination spot size. In one example, a reflective optics ellipsometer allows for a smaller spot size on the metrology target by reducing chromatic aberrations commonly associated with the use of refractive elements. Such a system is described by U.S. Pat. No. 5,608,526 entitled “Focused beam spectroscopic ellipsometry method and system,” issued Mar. 4, 1997, to KLA-Tencor Corporation, the contents of which are incorporated by reference as if fully set forth herein. In another example, a metrology tool employing an apodizing element is described by U.S. Pat. No. 5,859,424 entitled “Apodizing filter system useful for reducing spot size in optical measurements and other applications,” issued Jan. 12, 1999, to KLA-Tencor Corporation, the contents of which are incorporated by reference as if fully set forth herein. The apodizer provides a smoothly varying spatial filter to reduce diffraction tails in the illumination spot on the sample.
In general, it is often desirable to configure metrology systems with multiple angles of incidence and several wavelength bands in an attempt to achieve small measurement spot size. For example, metrology systems having multiple angles of incidence are described by U.S. Pat. No. 6,429,943 entitled “Critical dimension analysis with simultaneous multiple angle of incidence measurements,” issued Aug. 6, 2002, to KLA-Tencor Corporation, the contents of which are incorporated by reference as if fully set forth herein. In another example, metrology systems having several wavelength bands are described by U.S. Pat. No. 7,061,614 entitled “Measurement system with separate optimized beam paths,” issued Jun. 13, 2006, to KLA-Tencor Corporation, the contents of which are incorporated by reference as if fully set forth herein. However, in some examples, e.g., in composition measurements where it is desirable to perform measurements at oblique, near-Brewster angles of incidence (AOI), geometric scaling effects cause an undesirable enlargement of the measurement box size at large AOIs.
To shrink the size of the measurement box size the amount of signal information that arises from the area surrounding the measurement target and reaches the detector must be minimized. To minimize undesirable signal contamination, the illumination light must be projected onto the measurement target with a minimum of spillover outside of the measurement target area.
In the past, the illumination spot size was reduced by increasing the system NA and reducing the size of the illumination aperture (e.g., a polarizer slit). Although increasing the NA addresses diffraction induced spillover, increasing the NA increases aberrations, increases cost, creates optical alignment difficulties, and reduces the transmission efficiency of the optical system. Although reducing the size of the illumination aperture addresses geometrical properties of the spot image, the reduction in size of the illumination aperture increases coherence effects, reduces light throughput, and requires a tightening of optical system tolerances.
Despite existing approaches designed to control measurement box size, achieving a small measurement box size specification over the full measurement range is very challenging. This is especially the case at both large oblique angles of incidence (AOI), where the incident beam covers a larger area, and at longer wavelengths, where diffraction effects introduce significant limitations.
As lithographic and inspection systems are pressed to higher resolutions, measurement box size becomes a limiting factor in maintaining device yield. Thus, improved methods and systems for achieving a small measurement box size associated with a variety of metrology technologies are desired.